1. Field of Invention
This invention relates to enhancement of facsimile image quality for standard resolution images.
2. Description of Related Art
Facsimile machines generally image and transmit documents at a resolution of either approximately 200 dots per inch (dpi) horizontally and 100 dpi vertically (so-called standard resolution) or at 200.times.200 dpi (fine or detailed resolution). This practice is defined by the Group 3 standards. The detailed resolution offers higher image quality, but the standard resolution is used more frequently.
However, 200.times.100 dpi images are more difficult to read and are more susceptible to distortion than 200.times.200 dpi images. Nonetheless, the 200.times.200 dpi detailed resolution is slower and consumes more memory than the standard 200.times.100 dpi image. Thus, a need exists for producing high quality 200.times.200 dpi images with the speed and lower required memory capacity of a 200.times.100 dpi image.
There are two common methods for producing a 200.times.200 dpi facsimile image from a 200.times.100 dpi image. The first method uses simple scan line duplication that produces coarse results. This method is commonly used by facsimile machines to normalize a received standard resolution fax image. The duplication method simply applies the same value of the pixels in an adjacent scan line to the value of the unknown pixels.
The second method, one example described in McConnell and Bodson, in the book "Digital Facsimile Technology and Applications Handbook", which is incorporated herein by reference, uses fixed interpolation. The interpolation method uses an algorithm that only considers the pixels contained in the immediately touching scan lines in determining the value of an unknown pixel. Thus, the accuracy of determining missing pixels using the interpolation method is limited.
Consequently, the duplication and interpolation methods fail to produce significantly high quality images.